


Quint's Dark Sanster Collection

by QuintessenceA



Category: Undertale (Video Game)
Genre: Alternate Universe - Experimentation AU, Crying, Double Penetration, Dubious Science, Emotional/Psychological Abuse, Food Deprivation, I Don't Even Know, M/M, Mindfuck, Near Death, Non-Consensual Somnophilia, Rape/Non-con Elements, Sans (Undertale) Needs a Hug, Sans is Having a Bad Time (Undertale), Undertale Skeletons in Heat, Vomiting, W. D. Gaster Being An Asshole, or something like it
Language: English
Status: In-Progress
Published: 2021-01-18
Updated: 2021-02-28
Packaged: 2021-03-16 11:20:30
Rating: Explicit
Warnings: Rape/Non-Con
Chapters: 4
Words: 9,188
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28830309
Author URL: https://archiveofourown.org/users/QuintessenceA/pseuds/QuintessenceA
Summary: This is going to be where I drop the rest of my Sanster bits. It's all awful, cruel Gaster doing awful, cruel things to his creation.I wanted to write it. So I did.
Relationships: W. D. Gaster/Sans
Comments: 20
Kudos: 26





	1. Rougher Than Necessary

**Author's Note:**

> Gaster's experiencing a heat. He doesn't have time for his usual mind games, he just wants to clear it NOW.  
> ...ok, maybe a little bit of mind games, too.
> 
> (I'll come up with a proper title and description in the morning)

"Hold still!" Gaster snarled, pressing harder against S01's ribs. They creaked alarmingly, and S01 couldn't stop another pained wheeze that escaped his teeth.

S01 tried to nod his head, tried to explain that he was trying, he was *trying to be good!* as tears streamed down the side of his face. It was no use, the hand constructs held him too tightly even for that slightest movement. They were everywhere, wrapped around his arms, his femurs, his throat... There were too many, and they were squeezing tightly, far too tightly.

Gaster had never done this, never *attacked* him like this. Stars, he was out of control, and as the doctor roughly stroked his pelvis, trying to force his magic to form, S01 wondered what he'd done. Had he failed a test? Was the doctor angry with him? Did it have something to do with the strange smell in the air, thick and pungent enough that it seemed to coat the doctor's body?

It scared him.

Gaster's fingers moved to his pelvis, roughly stroking along the inside of his pubic symphysis. It was a move that had been bearable in the past, enjoyable even in a way, but not like this. It did, however, make Gaster's intent blindingly clear.

"There, there, yes…" Gaster breathed as S01's magic fell into place, wrapping around the doctor's fingers in his haste to form it.

It was too rough, too fast, and the fingers burned as they were removed. Hands pulled him to the edge of the desk, dragging him across the wood. S01 tried to stay limp, but it hurt, and he couldn't resist tensing against the hands that held him.

Gaster didn't seem to notice, apparently distracted by the challenge of removing his belt. His lip was curled in annoyance, and he was making sounds S01 had never heard from him before.

Summoning more hands, he pulled at the belt until it simply snapped. But there was no measure of victory in the doctor's gaze as he absolved himself of pants. He gazed down at S01, and there was a darkness in his eyes that promised pain.

S01 didn't know what to do. There wasn't anything he could do.

He began pulling at S01's arms and legs, twisting his femurs until they were spread wide apart, presenting him before Gaster's hungry gaze. They didn't stop there, hands tightening as the doctor devoured him with his eyes. The pressure grew until S01 was forced to cry out in pain.

"Stop that!" Gaster shouted suddenly, hand contructs tightening around S01's jaw. "Be quiet!"

They muffled his whimpers, but the other hands still pulled, his limbs spreading further still until he feared his hips would snap under the strain. Panicking, he cried louder, desperate to make the doctor stop before something irreversible happened.

Gaster ignored him, but the sound seemed to motivate him as he moved in between his legs. The constructs let go of his hips to be replaced with Gaster's own hands gripping S01's ilia, his fingers digging into the bone hard enough that S01 knew he'd be lucky if they only bruised.

When the tip of the doctor's cock pressed against his folds, he had barely a moment to draw in a breath before it forced its way inside of him.

He screamed anew as Gaster grunted above him, pushing deeper and deeper, tearing him apart. Tears streamed down his face, and he could feel himself bleeding. Gaster snarled again, a wordless sound of need, and the pressure within him grew past pain and dipped into something darker.

The smell was overpowering now, a cloying scent that filled the room. It was undeniably the doctor. It smelled like him. It tasted like him. It flooded around him, the scent filling his skull until it blocked out even the pain. He felt himself opening inside, felt his body giving way as Gaster pressed into him.

He couldn't focus on anything but the smell of the doctor, the feeling of being split open. His fingertips scratched against the wood, small, frantic twitches that were the limit of his mobility.

He'd never felt so violated, so utterly destroyed.

By the time Gaster finally hilted inside of him, S01 had run out of screams. He could do nothing but let out shuddering breaths, one after the other, against the hand pressed to his teeth. He couldn't even cry anymore.

"This-" Gaster growled, his breathing uneven as he began to rock his hips. "This is what you're good for! This is what you're made for!"

Hands clenched rhythmically around him as the doctor sped up, touching him everywhere. With a particularly jarring thrust, one slipped from his shoulder to his throat, and began to tighten.

S01 choked, his vision darkening at the edges. He could feel his consciousness slipping away, could feel himself dying.

Did Gaster even notice? Would Gaster even care?

Fighting against the heavy weight that pulled at him, he forced his eyes to stay open, looking to the doctor's face. Gaster was staring down at him, his mismatched eyes avidly watching his every expression. When he realized S01's gaze was on him, his smile crawled higher.

S01 found his tears again. After everything, the doctor didn't care. He was only a thing, to be used and discarded at the doctor's whim. If Gaster ruined him, broke him from the inside out, it wouldn't matter. He was replaceable.

As he cried, it felt like his resistance was pouring out of his eyes with his tears. He felt his body relax, abandoning any pretense of self preservation. The hand on his throat stayed constant, no longer squeezing but not loosening either.

Gaster leaned down then, and S01 couldn't find the will to do anything but stare as the doctor's face came closer to his. The hand lifted free from his mouth, and his jaw fell open, unsupported.

"Now you understand," Gaster said, his lips brushing against S01's mouth as he spoke. Then he kissed him.

S01 gave in, and let himself drown.

He breathed in Gaster, the smell of him, the feel of him. It was all S01 could do to keep breathing as he was rocked back and forth with each of Gaster's thrusts. The world fell away until it was just him and the doctor. S01 let his mind go, abandoning everything to the doctor's ministrations.

There was only Gaster.

Nothing else mattered as Gaster continued to use him, tearing him apart and putting him back together, only to tear him apart again. It was all he was good for, after all.

It didn't take long for Gaster to find his climax. He growled against S01's mouth, and S01 closed his eyes as he felt the doctor release inside him. The scent grew, making him feel heady, and combined with the sudden heat rushing through him, S01 could do nothing to stop himself from gasping as his body found its own finish. 

He opened his eyes again, but the world was dark. So this was drowning, was it?

He barely noticed Gaster pull out of him, too lost in his haze to care. There was a wetness against his legs, and the hands were gone. Without them holding him down, he felt like he might just drift away.

"…01." Gaster's voice seemed to be coming from far away.

Something was placed over him. A blanket? A coat? He was being bundled up, cradled even. The world seemed to be spinning under him. He was cold.

"A little food, you'll be fine. You're not realy that frail."

S01 nodded, unable to do any more. The doctor was right. The doctor was always right. Something was warm beside him. He sighed, curling against it, and let sleep take him over.


	2. Too Far, Discord experiment

**Summary for the Chapter:**

> "You don't have to be like this, you know."  
>  _silence_  
>  "Fine, fine! Have it your way! We're done for today. Now, just... summon your eyelights so I can make sure you're not hemorrhaging any magic."  
>  _silence..._  
>  "S-01? I am... not amused S-01. You need to stop this."  
>  _the silence continues, and Gaster reaches to place a hand on his arm. There's a slight tremor to the movement, if you look close enough_  
>  "Come now, get up. I'll get you cleaned and put to bed... just..."

**Notes for the Chapter:**

> The prompt was, basically, what if Gaster went too far with his experiment?
> 
> So over in K8's Sanster Discord I threw words at the screen for an hour, without a plan, just seeing where it took me. I'd never typed for a live audience before, and that was 95% awesome and 5% terrifying. Here's the result!
> 
> AND LOOK WHAT @E88_02 DREW AFTERWARDS!! This makes me so happy, I can't even properly say. I love, love, love it.  
> https://twitter.com/E88_02/status/1350729697764716544

Gaster draws S-01 closer, and there's the slight scuff of bones across tile. He still doesn't react. He starts to lift him by a single arm, that should be enough tension to elicit something! Only... it doesn't.

He uses both hands, and S-01 is light. Lighter than he should be. There's nothing but darkness in his eyes, his rib cage stilled.  
"01...?"

Lifting S-01 closer, he listens for breath. Silence.

Then, the panic hits.

He grabs frantically for a better grip on S-01's body, calling auxillary hands to hold him still. ~~Not that he can get any more still.~~ They turn his face to Gaster's, and there's nothing. Only the fact that he's not clutching a pile of dust shows that S-01 is alive.

He doesn't know how much longer he can last.

Clutching S-01 to his chest, he leaps to his feet. He's got to do something, something! He hurries down the hall, conjured hands throwing doors open as he runs at a dead sprint towards the medical lab.

He doesn't even wait for the hands to unlock the door, using a dozen hands to burst the barrier inward. It explodes from the frame with a sickening crack of wood ~~not unlike the sound of S-01's skull hitting the tile~~ , but there's no time to worry about details. 

Hand fill the room, pulling drawers and opening cupboards one after another as he searches. Somewhere, it has to be here somewhere! Then, he spots it. A red vial.

One of the auxillary hands grabs it, but it's shaking too much to get a decent grip. It falls to the floor, and Gaster feels his Soul stutter in his chest as it bounces against the tile.  
It doesn't shatter. It's fine. It has to be!

He rushes to it, dropping to his knees, S-01's tiny, tiny body rocking in his arms. He calls more hands, far more than he should need but he knows it's barely enough, to support S-01 as he reaches for the vial. He grips it, the glass smooth against his fingers but he doesn't drop it again.

Another hand bumps into his shoulder and he turns with a frantic glance. Smarter than the rest, this ine has brought him a syringe, called by his desperate need.

He takes it, and as quickly as he can plunges it into the red vial. The liquid swirls as he draws it out, glistening like living magic in the dim light.

The hands bring S-01 towards him, and he meets them halway. He takes the syringe... only to realize there's no where to inject it. There's no summoned magic. No where to inject the needle!

He scrambles closer with a short, pained sound as he reaches for S-01's head with one head. His thumb rubs under his jaw, desperate to draw a response. Maybe, oh stars, maybe...

Nothing! No, no, no! This can't be it!

"Please!" He whispers, pulling S-01's face closer, "Please, just a little bit, only a little...!"

Another hand rubs against S-01 jaw, and his mouth falls open. He places a hand against the inside of his teeth, rubbing softly. "Please, for me, summon your tongue."

There's a pulse of warmth against his fingers, a surge of magic to weak for him to even be sure it hadn't come from his own hands.  
He holds his breath.

A flash of blue, and S-01's tongue forms in his mouth. It's pale, sickly and nearly white with lack of magic.

But it's there.

Without wasting another moment, he draws it out.

"Good, good, yes, that's right, very good..." He says, unable to stop the flow of words as he slips the needle into his tongue. He depresses the plunger, and the magic fills with red. "Good, please, good."

He's trying not to shake as he pulls it out again. Once it's free, he tosses it to the floor and cradles S-01 against his chest. He rubs at the back of his head, his ribs, other hands petting his shoulders, his arms.

He can feel the warmth now, the Determination flowing through S-01's bones as it drags him from the edge.

Then he hears it. A small, choked gasp.

Against his side, S-01's fingers twitch, but he doesn't pull him away to look. He doesn't need to look, he doesn't want to see ~~what he did~~.

The twitching fingers begin to spasm, and S-01 breathing hitches as the warmth against Gaster's chest rises. 

"It's alright, S-01, it's alright now," Gaster says. The heat is getting worse.

Then, S-01 begins to scream.

Gaster pulls back then, giving S-01 space to breath. S-01 doesn't even seem to notice, his eyes screwed shut as his jaw hangs open, as if he could detatch it by sheer will alone. His tongue was tainted a deep red, pulsing slightly with the beat of his Soul.

The Determination! Stars, Gaster didn't even check the dosage!

"it hurts!" S-01 gasps, twisting and bending in Gaster's arms. "no, it- it- ahhh!"

Gaster pulls him back, using his extra hands to stop S-01 from snapping his own back. The heat is surging now, drops of red leaking from S-01's bones as the magic roils through him. There's nothing to do but ride it out.

"Shh, shh," Gaster whispers, using his true hand to cup the side of S-01's face. "Look at me, S-01. Open your eyes."

Shuddering under Gaster's fingers, S-01 opens his eyes. There's light finally, thankfully in his sockets, white shaking points that look on the verge of bursting.

"pl-please, don't, it hurts..." S-01 begs, his hands gripping weakly against Gaster's coat.

"I know," Gaster whispers back. "It will pass. Stay with me, S-01, just a bit longer."

Gaster tucks his other arm beneath S-01's back, and shifts until he's sitting on floor properly. S-01 barely responds other than to whimper in pain, but it's something. He's moving. He's alive.

As Gaster cradles S-01 shaking, trembling form, hands softly petting every inch of bone, it hits him. 

He'd gone too far. He'd almost lost him.

Slowly, the tremors pass. The heat, still intense, is no longer burning. S-01's whimpers have faded, but he's still breathing too quickly against Gaster's chest.

Gaster doesn't put him down.

Slowly, S-01's hand begins to move. He's not looking at Gaster as he reaches out, his phalanges stretching to touch the back of Gaster's hand.

When Gaster doesn't pull away, his grip tightens fractionally until he's clutching Gaster's hand in his own.


	3. A Brighter Darkness

**Summary for the Chapter:**

> A glimpse into a future of Behind Closed Doors. Experiment Sans (S01) had been rescued, but it's not the brightest timeline that it could be. Still, it's better than the experimentation universe.
> 
> At least, I think it is.
> 
> (Chapter will probably be absorbed into Doors series when the story reaches this point)

~~~

Scrolling through the previous day's camera footage, Gaster paused as he saw their 'guest' come into view on the screen. After a quick glance around the room to ensure he was alone, Gaster paused the video, zooming in for a better view. S01 had been walking from Gaster's office when he crossed the camera's path, trading for a new book to read while the doctor was out.

This was the most Gaster would allow himself, moving through the frames until he stopped on one where S01 was looking straight into the camera. The look of fear, ever present in S01's eye sockets, called to him, a deep wound begging to be ripped open, full of shiny emotions to play with.

"You want him, don't you?"

Gaster flinched at the sound of Sans' voice behind him, but he managed to hold back anything further.

"The thought has crossed my mind," he admitted, returning the camera to normal play.

"Yeah?" Sans asked with a small, knowing laugh. "Mine too."

There was a pause while Gaster considered that as they watched the screen together. When S01 stepped out of view, presumably retreating back to his room, Sans spoke up again. "What do you say you let me get him for you?" 

Gaster's eyes slid over to look at his capricious assistant, to find Sans staring at him with a deceptively lazy smile. 

"It's part of my job, right?" Sans asked. "Getting you the things you want? Helping you out?"

It was an insidious idea. With Sans' help he'd managed to curb his darker tendencies, though the desires never truly went away. It was why Gaster had kept himself away from S01, barely interacting and leaving the small monster's care to Sans and Alphys. He couldn't trust himself. 

It was wrong to consider such a selfish, indulgent idea, especially after what happened the last time he'd failed to restrain himself. He swallowed thickly, trying not to think of the broken expression he'd seen once in another Sans' eyes. 

He wanted it too much. He should say no.

"Are you sure that's a good idea, considering where he came from?"

Sans gave a short, surprised laugh. "Listen, you might be a little twisted with a few screws loose, but you're almost vanilla compared to his original Gaster," he said with a shake of his head. The smile he gave Gaster then was smug, a cruel edge to the way it spread across his face. "If you're worried, there's a trick to it. All you got to do is make it a hurt/comfort sandwich. He'll melt for you, no matter what you want to do."

"Sandwich?"

"Sure," Sans said with a casual shrug. "Comfort, hurt, comfort. Start off all good, give him some compliments, get his engine going. Show him how nice you can be. Then, break him apart. What is it you want? To make him cry?"

Gaster didn't answer, but Sans' smile sharpened and he stepped closer. His voice dropped an octave as he whispered, "Do you wanna hear him beg? Wanna hear him scream your name?"

Gaster took in a deep breath, letting it out in a long exhale.

"You can have those things," Sans said, his voice soft as a promise. "And then, when we're done with him, just finish him off with a few more nice words and he'll go right back to being yours."

"When 'we' are done with him?" Gaster asked, raising a brow at him.

"Sure. You don't think I'm going to let you have all the fun, do you?" Sans asked, his eye lights sharp with mirth.

"Ah, is this your price then? I thought you were being rather accommodating."

"Oh, you know how accommodating I can be," Sans said with a wink, and Gaster fought to keep from smiling, knowing that would only encourage him further. "But yeah," Sans continued, "that, and I want to see his face. I'm thinking that 'sandwich' could be more than just a metaphor, but we'll workshop the details later. Give me a few days, and if you could be a little extra... handsy now and then, that'll help a lot."

Gaster should turn him down. That would be the right thing to do. He took a deep breath-

"Think of it this way," Sans interrupted. "He's still way better off than he could be." His smile widened as Gaster's resistance wavered. "And you won't even have to feel guilty this time, because after I'm done with him, it's going to be his idea."

Well, Sans did say he would always be there to help him.

~~~

S01 stood outside of Gaster and Sans' office, tugging fretfully on the long sleeves of his sweater. He couldn't get Sans' words out of his head.

_"We all have to make ourselves useful around here. Well, I'm not talking about you, of course."_

Sans had been busy, distracted and annoyed with his work. It had been a slip of the tongue, S01 knew. Sans hadn't meant anything by it. 

_"Having you around means a bit of extra work, but that's ok."_

It was true, wasn't it? S01 ate their food, read their books, wore their clothes. And he hadn't given them anything back. Not that they had asked anything of him, but they knew he was a tool, meant to be used. Why would they keep a tool without a use?

_"Just because you were created to be a toy for the Doc doesn't mean you have to be one. I mean, I'm sure you were pretty good at it, but..."_

He remembered how Sans had shrugged, letting the sentence trail off before changing the subject. The trouble though, was that S01 knew it was what he was good at. It was what he was made for, and what made him useful. It was how he could be useful to them.

And it wasn't like the Gaster here didn't want him like that. Sans, too, by the way he acted. The way they would touch him sometimes, a hand on his arm, a gentle pat on his shoulder, gestures that always seemed to linger a moment longer than they had to.

It seemed like everytime he looked into the Doctor's eyes he saw hunger, the kind that back home would have meant S01 was in for a bad time. But… he knew that sometimes he'd been able to please his Gaster. In fact, it was the only way he'd ever been able to please his Gaster.

If he wanted to stay, he had to make it worth keeping him.

He knocked on the door.

"Come in," Gaster called, and S01 went inside.

Gaster was sitting at his desk, going over some papers with Sans standing beside him. They both watched as he entered, neither looking surprised to see him. Fidgeting with his sleeves, S01 stopped right in front of the desk.

Now that he was here, facing them, it was so much harder to speak up.

"i want to help you," S01 said quietly, twisting his sweater in his grip. "i want to give back for all you've done for me."

Sans looked over, trying to catch Gaster's eye but Gaster just smiled at S01. "What do you mean?"

"that… i want to be useful to you. i want to help and i- i can, by making you feel good." To make his point clear, he lifted up the hem of his hoodie. His magic had already been called to fill his pelvis, a reflex that came easily, knowing they'd be able to recognize the glow.

"You're offering… yourself?" Gaster asked, and S01 could hear the greed in his voice. He nodded, lowering his shirt again.

"How about that," Sans said, walking around the desk to join S01 across from Gaster. "I was just saying how stressed the Doc's been lately, and I've been too stressed myself to properly do anything about it. We could really use your help."

"i can do that," S01 said, looking up at Sans. 

"I know you can," Sans said, reaching out for S01's wrist. "Come'ere."

S01 couldn't help his flinch as bony fingers closed on his arm, but the hold was gentle as Sans pulled S01 close, bringing his other hand up to S01's face as Sans kissed him.

It was so unexpected, and S01 froze in response. Though he knew about kissing in general, S01 had never been kissed before, and he didn't know what to do. Sans didn't seem to mind, in fact, he seemed to be enjoying himself.

"what was that for?" S01 asked when Sans broke the kiss, surprised.

"You didn't like it?" Sans replied, his grin unchanging.

"i- i didn't say that," S01 protested, even as Sans began pulling S01 around the desk. 

"It's nothing," Sans said, rubbing a thumb across S01's wrist. "Maybe the Doc can help you better."

He was taken over to where Gaster sat, and Sans' pull turned into a push, offering S01 out to Gaster. Despite having asked for this, S01 began to shake in fear when Gaster reached forward, grabbing S01 around the ribs and pulling him into his lap.

He jumped as he felt a hand drop down to his iliac crest and he looked up to see Gaster smiling down at him. This close to Gaster, his hand on S01's hip… 

S01's resolve slipped. 

This Gaster wasn't as nice as he tried to pretend. S01 could see it. It was in the expression he wore when he thought S01 wasn't looking. It was in his eyes when he'd quickly excused himself the one time S01 had cried in front of him. It was in the way he looked at Sans sometimes, too. And though they'd tried to hide it, he'd more than once heard Gaster make Sans cry out in pain, seen the bruises left behind, and Gaster actually _liked_ Sans. S01 wasn't special to him, he was just… a tool.

Maybe… maybe he hadn't thought this through correctly.

Gaster leaned over him, taking his chin in one hand and tilting his face up, staring into S01's eye's as his other hand traveled to slip into his borrowed shorts. S01 began to squirm, trying to get away from the wandering hand, but he couldn't move in Gaster's grip.

"It's alright," Gaster said, "Relax. You're fine. Besides, you came to us, remember?"

He was right. S01 had agreed to this. He stopped struggling, forcing himself to to relax in Gaster's hold. The doctor's fingers crept into his pants, and… It didn't feel bad, not exactly.

Gaster's fingers were demanding but not cruel as they coaxed his magic to soften, readying itself for use. Though it wasn't painful, there wasn't enough intent in the touch to make it feel good. S01 brought his hands, still covered by his long sleeves, up to his mouth and began to gnaw on the edge of the sweater as Gaster worked one finger, then another inside him.

The sweater muffled S01's small whimpers, and he bit down harder as Gaster forced his fingers in deeper. It was uncomfortable, but he could feel it working, his body growing slick under Gaster's touch.

"Now, now, I know it hurts a little," Gaster said in a soothing tone, "But you're going to need to relax if you expect to take us both."

Both?

The third and fourth finger pushed in alongside the others, and S01 let out an noisy exhale through his teeth, his frail body jerking against Gaster's hand as he moaned. Gaster soothed him, rubbing his back as he rocked against the fingers inside him. He could feel a wetness on his face, and realized that he was crying. Gaster noticed it too.

"Beautiful," Gaster said, only to move his eyes to Sans.

S01 followed his gaze to see Sans hopping up to sit on the desk, having already removed his short in the process. "I think you've got him ready for us," Sans said, sounding eager.

S01 could do this. He'd help Sans, then he'd help Gaster. It didn't matter that he didn't want this. Sometimes you just had to do what needed to be done. 

"Let's try to be gentle, shall we?" Gaster said, slipping his hands under S01 sweater. The touch was jarringly familiar, intent shifting to a demand that S01 couldn't fight as Gaster began to carefully remove S01's sweater. He wanted to complain, to beg them to let him keep it on, before reminding himself that this was what Gaster wanted. 

Gaster's hands reached under his ribs and legs, sliding off his shorts before lifting him effortlessly into the air. He was brought to where Sans' was sitting, and together the monsters maneuvered S01 until he was kneeling, straddling Sans' pelvis with his toes hanging off the edge.

Sans was ready, his cock hard and waiting.

S01 felt fingers hook under his femur spreading his legs and lowering him closer. He bit his tongue so hard he could taste blood, trying and failing to come up with any sort of defense. Isn't this what he wanted? To make Gaster happy?

If he was good, they would love him. They would keep him. He would have a home here.

He heard the familiar sounds of clothes being undone behind him, and he squeezed his eyes shut. Oh, stars, was this what they meant by 'both'?

It was too late to back out, they were going to do it whether he wanted to or not. There was nothing he could do to stop it. He put his hands on Sans' shoulders and tried to relax, taking a deep breath as he felt Gaster press up behind him at the same moment he was forced down against Sans. 

Tears sprang to his eyes as the hands on his femurs pushed him down, driving him onto Sans' length. He cried out with the shock of it, a high counterpoint to Sans' low moan. There was barely a moment to adapt before the doctor pushed himself in from behind. He let out an anguished cry as the thrust forced Gaster inside of him. It felt like a round peg being jammed into a triangular hole, and S01 struggled, pulling against Gaster's grip.

"wait! w- wait!" His voice cracked on a keening cry as the pressure built within him. There was no way he was going to be able to handle this.

Before him, Sans breathed out a pleased exhale, reaching one hand up to the side of S01's ribs while he supported himself with the other. 

"Is this too much for you?" Gaster asked quietly, leaning forward. It pressed him into Sans chest, pinning his arms as he was crushed between the two monsters. 

He was trapped. They were going to rip him apart. 

"y-yes," he said, forcing himself to try falling limp, dropping his face into the crook of Sans' shoulder.

"Ah, none of that," Gaster said, chuckling. He slipped his hand up under S01's chin, gently pulling his head back until S01 was forced to look up into the doctor's face. "Come now," Gaster whispered in his ear. "Don't you want to be good for us? You feel so good, so tight around me. Don't you agree, Sans?"

"Mmh," the other hummed, not bothering to hide his enjoyment.

Gaster let out a shuddering groan as he began to pump his hips, moving inside of S01 at an excruciatingly slow pace. Every thrust forced out a pained moan as S01 grit his teeth, tears welling in his eyes. S01 tensed, his body seizing up as they both filled him to the brink and beyond. It took all of his concentration to not outright sob as the two monsters used his body.

Below him, Sans groaned in pleasure as he lazily rocked up into S01. The smaller skeleton was doing almost nothing, leaving all the work to be done by Gaster and his larger frame, rocking S01's body back and forth along his length as Sans' hungry gaze watched S01's every expression.

"You look so good like this," Gaster said, his voice low and thick in S01's ear. "Doesn't he look good like this, Sans?"

S01 shuddered, warmth filling his soul as Sans hummed in agreement. "He sure does. A pretty little toy for us to break."

S01's breath hitched at the reminder. He wanted to please them. He wanted them to love him.

"Easy," Gaster said, dropping his hand from S01's throat to rest on Sans' leg.

"Or we could do it harder," Sans suggested, punctuating his words with a harsh jolt of his hips. A small whimper escaped S01's throat. He hated himself for it. He'd asked for this.

"It'll only get worse, you know," Sans said, sliding his hand to rest on S01's back. "Better brace yourself."

He wasn't sure how he could brace himself for something like this, but he took a deep breath as he felt himself being pressed down again, to slide deeper onto both of their cocks. They'd only barely started before, and now the real challenge was beginning. A tearing pain cut through him that seemed to be intensifying with every passing second.

He grit his teeth, squeezing his eyes closed as he let out an agonized whine. The pain was growing, and he didn't know how to handle it. His hands gripped tightly to Sans' jacket, as if that was the only thing grounding him to this world. That, and Gaster's voice in his ear.

"It hurts, doesn't it?"

He could only manage a nod.

Sans gave a sudden thrust upward and S01 cried out in sharp pain. S01 could feel himself being forced opened, held tight by both monsters as they did as they pleased with his body.

"Be gentle," Gaster warned.

"Nah," Sans countered. "Trust me, I know what he can take."

Another, more violent thrust from Sans sent a shock of pain through him and a true scream was forced out of his mouth until he bit his tongue to stop it. The move did nothing to stop the pained wheezes that continued to hiss out with his every breath. Gaster shuddered behind him, picking up his pace.

"He wants this," Sans said, his grip tightening as he set a speed to match Gaster's. "Sing for us, little bird."

He was so full, one monster pulling out as the other pushed in, never empty for a moment, while each push left him feeling on the brink of being split apart. The tearing sensation grew worse, and he knew he must be bleeding.

S01 couldn't take it anymore, and he began to scream.

He couldn't stop himself as they forced the sounds out of him, each scream getting louder than the last. His feelings of pain and terror were to strong to contain, spilling out from him in a torrent.

"I don't think I'll ever get tired of that noise," Gaster said, leaning in close to S01's face.

"Me neither," Sans said, his eye lights hazy as he stared at their writhing, screaming prey.

The pain was just getting stronger and stronger, until it felt like his very being was being torn into.

"Beg," Sans commanded, his voice cutting through everything, demanding to be answered.

"please…!" S01 cried, with a deep, gasping sob. "stop…! please… I beg you, i can't..."

"You can," Gaster said, his voice cold and merciless. "Say it again."

"please…!" He moaned, his voice already growing hoarse. "don't, please-!"

S01 screamed as he felt his magic being stretched, felt it being pushed further than it was ever meant to go as Gaster held him down, pressed him tightly together with Sans.

"Do you really want us to stop?" Sans asked, slowing his movement, and Gaster follow suit, picking up on some cue from Sans. "Say you want us to let you go and you can leave. You don't have to stay with us."

The temporary respite made S01's head spin. He hurt, his body throbbing with bright, stinging pain where they were sliding into him. He wanted to run, to escape. He'd made a mistake. Could it be true? All he had to do was speak up, and-

"Or," Sans continued, his voice as solemn as a prayer, "say that you want to stay. With us, right where you belong."

His terrified mind latched on to the last word.

"belong?" He whispered, 

"You belong with us," Gaster said, nuzzling the crown of his skull even as he slowly continued to move in and out of S01's abused body. "We'll take care of you. Stay."

S01 was shaking, his body trembling from the force of the emotions within him. They wanted him. They wanted him to belong with them. Could it be worth this pain?

Of course it was worth it.

"stay…" S01 repeated quietly closing his eyes as he felt tears streaming from his eye sockets, dripping from his jaw.

"See Doc?" Sans said, and the approval in his voice was somehow more terrifying than his cruelty. "He'd let us break him."

The truth of the words stung greater than the feeling of being torn apart. He would, he'd give them anything. Even if it killed him. He didn't matter anyway.

"Would you?" Gaster asked, his voice low.

S01 choked, stifling another cry as he gave a single nod.

"Do you want to be broken, my sweet little thing?" 

The nickname rolled off Gaster's tongue like the sweetest poison as Gaster reached a hand up to grip one of S01's false ribs. His thumb pressed against the bone hard enough to make it creak warningly, and S01 resisted the urge to twist in his grasp. Whimpering, S01 tipped his head forward, resting his skull against Sans' shoulder again as he struggled to simply breathe.

"Look at you," Sans said, softly petting the back of S01's skull. "Scared as you are, you're still here."

He wanted to melt into the touch. Soft hands, sweet words… Why did they only come with pain?

"I asked you a question," Gaster reminded, and a sharp jolt shot through S01's ribs as Gaster pressed the bone nearly to the point of breaking.

"n... no," he whispered, too afraid to dare stay silent, even as he knew that it wouldn't matter what he said.

If they wanted to, they would break him anyway. It was just something he had to accept. But it didn't mean he wasn't afraid.

"I'm sorry, I didn't quite hear that," Gaster said, and Sans' chest rumbled with a laugh under S01's forehead. "What was that?"

"i don't... please don't break me…" he said, a little louder. "please…"

A laugh left Gaster's lips.

"Ah, but it would be so easy..." Gaster crooned.

"It would," Sans agreed, his voice soft.

"I could break your bones one by one," Gaster said, his fingers moving upwards to press against each of S01's ribs in turn. "Watch as you try to crawl away before I grab your limbs and drag you back, hold you down, keep you from escaping."

Fear shot through S01. Not even his own Gaster would be that cruel. He pulled back, looking into Sans' face, desperate to find the lie in his expression. The monster's eyes were intent on his, dark and merciless.

He whimpered, and Gaster's hand slipped back against his throat, pulling S01 against his chest. The fingertips teased, dragging softly against his vertebrae as Gaster spoke.

"I could break you now. It would be so easy. All I have to do is tighten my grip."

The hand wrapped around his throat squeezed, Gaster's fingers completely encircling his neck.

"Or maybe we should just pull your soul from your chest," Sans suggested, adjusting his grip to press his hand flat against the middle of S01's chest. There was a light tug against his soul, and S01 froze solid, scarcely able to breathe. "Just pull it right out and let you watch as we squeeze the life from you, drop by drop."

"no..." S01 mouthed in horror, unable to voice the word. He wouldn't be able to take it. He wanted to run, to hide, to save himself from the monsters.

But he stayed still. He couldn't bring himself to move. He'd offered everything, and now they were going to take everything. He was paralyzed with fear and terror as the two who were supposed to protect him prepared to kill him.

Then he felt Gaster press a kiss to the top of his head.

S01's voice returned, misery overtaking him as he began to wail in despair. Did they want to kill him or love him? He didn't understand, and that hurt in way that his mind struggled to comprehend. He wanted them to want him. He wanted to belong with them, and they said he did, but they were breaking him instead. He would never be fixed.

"You really are ours," Gaster said, "and we're going to keep you."

Sans laughed, slipping a hand behind S01's sacrum and thrusting in a harsh rhythm that slammed S01's pubic bone against his own. He screamed, but the monster continued, drawing it out, and Gaster soon joined to match it.

"He knows who he belongs to, who the real monsters are," Sans growled, his thrusts growing faster and more brutal than before. "He knows what he's good for, don't you?"

"yes!" S01 shouted, sagging in their hold.

The thrusts were hard, and they hurt him. He didn't bother to temper his voice anymore, freely crying out as the tears continued to fall. Every thrust was painful, every single one. He wanted it to stop, he really did, but even stronger was the need to be what they wished him to be. 

Gaster began to whisper next to his ear. 

"Listen to you cry. Poor little thing. You're nothing. You'll never be anything, and you know it," he said.

Every word pierced into S01's mind like daggers as Gaster continued to hurt him, over and over again. It was too much like what he'd heard before.

_"This is what you deserve. You're nothing but what I made you to be."_

S01 could feel himself growing light-headed as he panted for breath. Sans' hand was on his spine, staring into his eyes and giving S01 nowhere to hide.

"Say it," Gaster snarled, giving him another hard thrust and driving the breath from him. "Say it or I'll never stop."

"i'm n…, i'm not…," he gasped between sobs, hating the way his voice shook.

"What?" Gaster demanded. It hurt so much, and it kept hurting more and more as they both continued to slam into him over and over again.

"please…" The tears wouldn't stop, falling from his eyes as he fought to keep himself together, just a little bit. He wanted it to be over. "please, just this, please don't..."

"Give him what he wants," Sans said, his voice low and husky as he looked over S01's shoulder at Gaster. "Give him what we both want."

Sans' hand left S01's back, and he felt the cold fingers trace down, curling around his lower spine to hold him still as Gaster began to lose himself to the pleasure.

"Tell me," Gaster demanded. "What are you?"

There was no choice. He'd been foolish to think that he meant anything. This was what he was for. This was what Gaster wanted.

And what he'd agree to.

"i'm nothing!" He yelled, the sound high-pitched and desperate as he felt his body tensing up, as if fighting his own admission, as if he wasn't in pain enough already. "i'm nothing! please! gaster, please! no- no more! g-gaster!"

"Yes… good boy, good…" Gaster groaned, shuddering as he pressed himself deep into S01's body. He continued to whisper sweet, filthy praise as he came, but S01 could barely hear him.

He sounded so far away, even though his voice was right by S01's ear. 

As he felt Gaster pulse inside him, S01's head was pulled up to face Sans, and the monster kissed him deeply. Sans' other hand snaked into S01's ribcage, and before he could even process the move his mind was slammed with pleasured intent as Sans' fingers closed around his Soul.

He cried out against Sans' teeth, filled with an intensity unlike anything he'd felt before. He could feel himself getting close, pleasure tearing through him at a blinding speed. So close, so close, so close-

"Come for us," Sans commanded.

His voice was hoarse and low, and it sent a shiver down S01's spine, dragging him down with it. S01 came with a high-pitched whine, trapped between the two monsters. 

Sans continued to kiss him, swallowing down his cries until S01 felt him come with a harsh, wracking tremor of his own. Sans grunted, pulling him even tighter into his arms until he finished. Finally done, Sans released him.

S01's arms fell slack to his sides, and it was only Gaster's hold on him that kept him upright.

"There, there," Gaster said, and S01 couldn't deny the weak spark of satisfaction at the contentment in his tone. 

He didn't put up any resistance as he was drawn into Gaster's arms. An errant glance at his own pelvis showed it to be a perverse mess of Gaster's purple, Sans' blue, and a concerning amount of his own torn magic. He quickly looked away, to find Sans watching him.

Sans looked well pleased. 

"You did good, shorty," he said, smiling as he regained his clothes.

"i did?" S01 asked weakly, turning his face to look at Gaster. 

Gaster was smiling, and there was no sign of cruelty in the expression. "Yes, you did," he agreed easily.

Battered, bruised and soul weary, S01 smiled back. He'd done it. He'd made them happy. He could stay.

"Hey G," Sans called, and Gaster looked up. S01 didn't, too caught up in Gaster's smile. "How about you get him taken care of and I'll whip something up in the Lab kitchen? Got this easy pancake recipe from Pap's sitter I think I'll try making on a hotplate."

"That's a wonderful idea," Gaster said, looking back down at S01. "What do you say? Would you like some pancakes?"

S01 nodded, his eyes still on Gaster's face. He looked so happy now, not a trace of the monster that had threatened to break him apart piece by piece mere minutes ago. It had to be worth the pain. He was going to get pancakes, at home, where he belonged. He was useful to them. It didn't matter how much more intense the pain was, how devastating it was to hear them speak to him like they had.

This was the better universe.

…right?


	4. Weak

**Summary for the Chapter:**

> Gaster needs some stress relief, but S01 isn't in any state to help him.
> 
> Based on a Twitter convo (twitter.com/K8Maybe/status/1343662857410539523), plus a few other ideas from Discord
> 
> Besides the normal "Gaster's a horrible person", (ie noncon) warning, this one has headcanons of severe food deprivation of a skeleton, vomiting and... idek how to tag it. Noncon somno adjacet?

Failure! One test after another, all failures!

Gaster mutters bitterly to himself as he stalks down the hallways of his lab. At least he has one experiment in this stars-forsaken laboratory that wasn't a complete waste of his time. And if there's one thing in particular that S01 is good for, it's stress relief.

He opens the door to S01's cell.

The skeleton doesn't look up from his sleeping mat when he enters. 

"S01," Gaster calls, but to his irritation, S01 doesn't react.

Sleeping. How typical.

"S01, wake up!" He snaps, letting the edge of disapproval slip into his voice as he walks to the edge of the bed. S01 doesn't move. Looking closer, Gaster notices the bones are pale, the magic between the joints dim and nearly colorless.

Gaster pauses, surprised. This was unusual.

When he leans down, feeling for the tingle of magic in the mana line parallel to his cervical vertebra, the skeleton's eye sockets flutter but they don't open. It was only then that Gaster realized he'd hadn't been to this side of the Lab since he'd started his latest project.

When was the last time he'd left S01 any food? Two days ago? Three?

Brushing his fingers down the side of S01's still face he decides that it doesn't matter. He's too pent up, he needs relief.

Clearly, S01 is in no condition to walk. Gaster will certainly have to do something about his failing magic levels, but first…

He scoops S01 into his arms. It's easier this way, to simply carry him. It's surprising how cold S01 is to the touch, and it's almost enough to deter his interest, but he can't ignore the way his magic thrums with want as he pulls S01's body closer. Stars, he needs this.

Uninterrupted, it's a short walk to his office. S01 hardly responds as Gaster lays him across the desk, but his bones rattle pleasantly as his short gown is removed. When Gaster begins to rub his fingers against the notch at the front of S01's pelvis, there's finally a response.

S01 gives a small, sharp inhale and his head falls to the side. Good, not too far gone after all. That makes this easier.

"It's been too long," Gaster says, feeling S01's hips twitch feebly, once, twice, as he continues to rock his palm. "You probably need this as much as I do..."

S01's head shakes to the other side, a plaintive whine escaping his teeth.

But he's not unaffected. Magic begins to swirl in his pelvic cradle, slowly but inexorably called by Gaster's intent. It's a damp, wispy mist that barely has substance enough to be felt. 

There's not much there, but there doesn't need to be. 

As Gaster curls his fingers around S01's pubic symphysis, gently stroking the backside, S01's fingers twitch against the desk, aborted little gestures that make a soft scratching sound. It's the most movement he's made since Gaster found him, and coupled with the sluggish response to Gaster's intent it almost feels like resistance. Frowning, he reaches his free hand up to press against S01's sternum. 

"Do it," he commands.

A shudder runs through S01 body as enough magic finally reaches his pelvic inlet, and an entrance forms. It's dim, lacking the vibrant color it should have, and looking nearly grey in comparison. S01's small body gives a final twitch as the construct settles before he falls deathly still.

Gaster continues to stroke his fingers against S01, trying to encourage more slick to gather, but S01's magic stores seem to be more or less depleted. A pity.

It hardly matters. There's enough for Gaster's needs.

He draws S01 to the edge of the desk, letting his thumbs stroke against the chill bone. S01 is as pliant as a doll.

"You're so pretty," Gaster says, reaching to stroke S01's cheek with a finger. "Do you know that?"

S01's eyes remain closed, and he doesn't answer.

"You always make me feel so good," Gaster whispers, undoing his belt and pulling his cock free. There's no reaction, no sign of life other than S01's disquietingly shallow breathing. His lovely, perfect little doll, Gaster thinks as he lowers his head, listening to the subtle beat of S01's soul.

Gaster begins to slide himself against S01, letting his own precum lubricate the way as his cock begins to infiltrate the thin, weak magic. He leans in, drawing an arm beneath S01's body to pull him close.

With a firm thrust, he enters S01's folds. They part around his girth, and he lets out a quiet moan of pleasure. S01's body may be cold, but he's pleasantly warm where it counts. Gaster pulls back and thrusts again, and again, and soon he's found his rhythm. 

Their coupling is nearly silent, no sound from S01 other than the wet noise of their bodies sliding together. Gaster's hand slips down between them, and he begins to rub his fingers against S01's magic in time with his movements. Gaster's other arm holds him close to his chest, keeping S01 from falling back.

After a few good thrusts he feels S01's body shiver in his grasp, and then S01's eye sockets flutter open. He stares into Gaster's eyes, his own eye lights dull and faded, the corners of his sockets pinched at the edges with his distress.

Gaster thrusts into him again, and then runs his tongue along the side of S01's face, pleased at the almost flinch it wrings from the skeleton. He whispers, "Do you like that?"

S01 doesn't answer.

Gaster smirks, then says, "You do, I can feel it."

He rubs his fingers against S01's clit again, a little rougher this time. S01's body shudders at the touch. 

To Gaster, there are no secrets when it comes to S01's body. He knows his experiment inside and out. Knows what he likes, knows what he hates. Knows that he would surely be crying at this moment, if he could. 

He leans in to whisper into S01's ear, "You're going to come for me."

There's a small sound, a faint, exhaling wheeze from S01's teeth.

"You can't help yourself, can you?"

Gaster's thrusts grow faster. S01's body begins to shudder, and he lets out a strained hiss. His whole body tenses, then releases as he reaches his apex.

"There you go," Gaster says, tightening his grip. "Just like I-"

He cuts off as S01 seizes in his grip, only to suddenly vomit against Gaster's chest. 

The acrid magic gushes out onto Gaster's labcoat, and he pulls back with a yelp, letting go of S01. The small skeleton collapses to the desk, a trail of bile leaking from between his teeth. Now separated from Gaster, his magic instantly dispells and there's another wet splatter as the little moisture he'd managed to gather hits the desk. 

Livid, Gaster stares at the anemic pool of pale blue sick, then up to S01's face.

S01's eyes are hollow pits of black, staring sightlessly into the distance. He looks spent and weak, fragile in a way that Gaster has never seen before.

As Gaster looks over the small creature, the sheer woefulness of the thing saps him of his rage. Instead, he just feels something that strays very near to being pity.

"I suppose I could have forseen this," Gaster says, grumbling to himself as he readjusts his pants, before pulling off his labcoat. "It's not exactly a surprise that nausea is a symptom of food deprivation."

He tosses the soiled labcoat aside, then grabs a hand towel from the low cabinet beside his desk. He wipes himself the rest of the way off, before reaching down to clean off his desk, and then, finally, S01.

"I'll just have to keep a closer eye on you," he says, as he brusquely wipes the vomit from S01's teeth. "Clearly you don't have the constitution required to withstand a few days without food. Your low hp, I imagine."

Once he decides that S01 is decently clean, Gaster pulls a package of instant noodles from his inventory and crushes them up in the bag. Once they're small enough, he tears a hole in the top of the plastic.

"Open wide," he says. 

Predictably, S01 doesn't respond. Gaster feels the slightest twinge of concern as he draws open S01's mouth, and shakes some of the noodle bits inside. He sits patiently, waiting the couple of minutes it takes for S01 to eat. It seems to take much longer than it should, but eventually the food dissolves, absorbed into S01's magic.

"There," Gaster says, feeding him a few more pieces. "And you'd better keep this down. If you were to vomit at this stage, I suspect you'll sustain actual damage and, frankly, that would do neither of us any good."

Gaster continues to talk as S01 eats the rest of the noodles without incident. By the time he's done, S01 has regained enough magic to respark his eye lights.

"Can you move?" Gaster asks, dropping the noodle bag in the trash and retrieving S01's smock. "Once you're back in your cell I'll give you some nutrition bars, and that should be enough to get you back to baseline normality."

S01's body trembles as he tries to move. He clearly can't muster the strength, and instead just lets out a low moan of distress.

"I assume that means no," Gaster says with a sigh, draping the smock over his arm. "Alright, I'll carry you. Try not to be sick again."

As gently as if S01 might break, which was still a possibility, Gaster picks S01 up from the desk, and cradles him in his arms. S01 doesn't resist, but there's a slight heft to his body that hadn't been there a few minutes ago. 

Perhaps he'd let S01 go a little too long unfed after all.

"I'll have to be sure not to let it come to this again," Gaster says, carrying S01 out of his office and towards the cell.

To Gaster's surprise, S01 doesn't even attempt to grab his coat this time, despite how much Gaster knows he loves to. Still too weak, he supposes. Instead, S01 gathers a breath, and speaks.

"...thank you."

Gaster ignores him. It's the easiest thing to do.

**Notes for the Chapter:**

> Gaster, don't starve your experiment, that's just not nice.


End file.
